The objectives for the single-dose study are to evaluate the safety, tolerance, clinical response, immunogenicity and pharmacokinetics of a single dose of placebo, 5, 10 or 20 mg/kg of cA2 in patients with active rheumatoid arthritis (RA) who are receiving methotrexate. This is a Phase II multi-centered, placebo-controlled, double-blind, ,4-arm parallel trial to evaluate a new treatment for RA. RA is usually treated with non-steroidal anti-inflammatory drugs and slow-acting anti-rheumatic drugs. These drugs are felt to decrease arthritis by reducing inflammation and suppressing the immune system. A chimeric anti-TNF antibody has been developed to neutralize the inflammatory effects of TNF that is found in rheumatoid joints.